Projection display system

ABSTRACT

A liquid crystal display system comprising: a. a dichroic mirror ( 2 ) arranged to direct first and second spectral components along a first processing path (A) and to direct a third spectral component along a second processing path (B); b. a first cubic polarizing beam splitter ( 4 ) arranged to direct the first and second components towards first and second liquid crystal on silicon (LCoS) panels ( 18, 22 ), respectively, and to direct the first and second components, after modulation, along a third processing path (C); c. a second cubic polarizing beam splitter ( 6 ) arranged to direct the third component towards a third LCoS panel ( 28 ), and to direct the third component, after modulation, along a fourth processing path (D); d. a cubic dichroic beam splitter ( 8 ) arranged to direct said first and second components and said third component along a fifth processing path (E); and e. three post analyzers (30, 32, 34) arranged on an external output face of the dichroic beam splitter.

This invention relates to a projection display system, in particular acolor projection display system.

Liquid crystal projection display systems are known. Liquid crystal onsilicon (LCoS) display panels are used to separately modulate threedifferent spectral components (red, green, blue) of a white light beam,and the three components are combined to form the output beam which isprojected on a display screen. LCoS projection display systems have theadvantage of a relatively high resolution at relatively low cost. Suchsystems are proposed for use in products such as large-screen desktopcomputer monitors, high-definition television (HD-TV) andhigh-resolution front projectors.

Various systems are known and proposed for use in separating the whitelight from the light source into the three separate components, and torecombine the beams after being modulated by the display panels. Oneexample uses cubic beam splitters and polarization optics in order toperform the separating and recombining operations. Such a system isdescribed, for example, in European Patent Application EP-A-1081964. Thepolarizing beam splitters separate the light into its separatecomponents and analyze desired parts of the three different componentsof the beam. In order to enhance contrast in the image produced, plateanalyzers, in the form of polarizers, are placed between the cubic beamsplitters, to additionally analyze the beams, and thereby enhancecontrast. These analyzers generate heat within the system. Therefore, ifthe arrangement is used at high brightness levels, problems such asthermal degradation of the analyzers and thermally induced stressbirefringence appearing in the beam splitter cubes occur within thesystem.

It is an object of the present invention to provide a projection displaysystem capable of operating at high brightness levels whilst providingimproved contrast in the output image.

In accordance with the present invention, there is provided an opticaldevice for processing radiation, said device comprising:

-   -   a radiation input means arranged to direct first and second        spectral components along a first processing path and to direct        a third spectral component along a second processing path;    -   b. first polarization-selective reflective means arranged to        reflect the first spectral component and the second spectral        component selectively in dependence on polarization states        thereof, to direct said first and second components towards        first and second radiation modulation means, respectively, for        modulation thereby and to direct the first and second        components, after modulation, along a third processing path,        said first and second components having different polarization        states when travelling along said third processing path;    -   c. second polarization-selective reflective means arranged to        reflect the third component selectively in dependence on a        polarization state thereof, to direct said third component        towards third radiation modulation means, for modulation thereby        and to direct the third component, after modulation, along a        fourth processing path;    -   d. spectrally selective reflective means arranged to process        said first and second spectral components similarly when in        different polarization states, and arranged to direct said first        and second components and said third component along a fifth        processing path; and    -   e. radiation output means arranged to process radiation along        said fifth processing path, said output means including        spectrally selective polarization-sensitive means arranged to        process said first and second components differently when in        different polarization states.

By using the spectrally selective reflective means arranged to processthe first and second components similarly when in different polarizationstates, and the spectrally selective polarization-sensitive meansarranged to process the first and second components differently when indifferent polarization states, the use of analyzers between thereflective elements can be avoided. An output analyzer can be placed onan external face of the device, thereby improving contrast whilstavoiding excessive heating of the device between the reflective elementswhen operating at high brightness levels.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, made with reference to theaccompanying drawings, wherein:

FIG. 1 is a cross-sectional view of a liquid crystal projection displaydevice in accordance with an embodiment of the invention; and

FIG. 2 is a cross-sectional view of a liquid crystal projection displaydevice in accordance with another embodiment of the invention.

FIG. 1 shows a liquid crystal projection display device in accordancewith an embodiment of the invention. A colour video display system inaccordance with this embodiment of the invention includes the displaydevice along with a radiation source, not shown, emitting substantiallywhite light in the form of an input beam (I) and further opticalcomponents, not shown, typically including a magnifying output lens forprojecting an output beam (O), and a projector screen for displaying thevideo image. The colour modulation device includes elements forseparately modulating different spectral components, each coveringsubstantially exclusive wavelength ranges in the visible spectrum, ofthe radiation beam, typically red, green and blue components.

A spectrally-selective reflection element, in the form of a dichroicmirror 2, is arranged at 45° to the input beam I. The mirror acts as abeam splitter, splitting the input beam into different componentstravelling along two orthogonal processing paths A and B. First andsecond components of the radiation beam travel along the firstprocessing path A into a cubic polarizing beam splitter 4, whereby thetwo components are separated and subsequently combined after separatemodulation. After separate modulation, the two components are projectedalong a third processing path C towards a cubic dichroic beam splitter8, through which the two components are transmitted.

A third component of the radiation beam travels along the secondprocessing path B into cubic polarizing beam splitter 6, whereby thethird component is projected for modulation. The third component is thenprojected along a fourth processing path D into dichroic beam splitter8, where the third component is reflected through 90° and therebycombined with the first and second components and projected along afifth processing path E towards an output path of the device.

In the first processing path A, the two beam components are subject tooptical processing before entering the cubic beam splitter 4. Apolarizer 10 cuts out all polarization components except for a singlelinear polarization component of the spectrally-filtered input beam. Theinput beam I itself is preferably substantially polarized, with thepolarizer 10 being arranged with its axis of polarization parallel tothat of the input beam.

A spectrally selective retardation plate 12 is used to selectivelyrotate the polarization state of one of the two components travelling onthe first processing path A. The retardation plate 12 could be of thetype which is available from the company ColorLink Inc, Boulder, Colo.

As a result of the selective retardation of one of the components of thebeam, one of the components is reflected by the polarizing beam splitter4 whereas the other is transmitted on the basis of the difference inpolarization states.

The first spectral component travels along a first separate processingpath F towards elements at which the beam component is modulated withthe appropriate part of the image signal. The first component isprocessed using a bandpass filter 14, a skew angle compensator in theform of a quarterwave plate 16 and a liquid crystal on silicon (LCoS)light modulation panel 18 which selectively modulates the polarizationstate of different parts of the first component on reflection inaccordance with an applied first colour component of the image signal.Selected, desired, parts of the component beam, distributed across thesurface of the LCoS panel 18, have their polarization state rotatedthrough 90° on reflection, whilst the remaining parts remain unaffectedon reflection. The desired parts are transmitted through thepolarization beam splitter C, whilst the unwanted parts are reflectedback towards the input light source. Thus, the polarizing beam splitter4 acts as a first-stage analyzer for the first component beam followingits modulation by the LCoS panel 18. Since the polarizing beam splitter4 is located immediately adjacent the LCoS panel 18 and its associatedlaminar components 14, 16, a relatively high degree of contrast isobtained during the first analyzing stage.

The second radiation component, which is transmitted through thepolarizing beam splitter 4, travels along a second separate processingpath G to be separately modulated. A skew angle compensator 20 in theform of a quarterwave plate processes the beam before the beam reaches asecond LCoS panel 22, at which the beam is modulated in polarization inaccordance with a second colour component image signal. On reflectionfrom the LCoS panel 22, the desired parts of the beam, which are rotatedthrough 90° at the LCoS panel 22, are reflected by polarizing beamsplitter 4 to join the desired parts of the first beam component alongthe third processing path C. Again, polarizing beam splitter 4 acts as afirst-stage analyzer for the second beam component. However, since thepolarizing beam splitter 4 analyzes the second component by reflection,the analyzing of the second component is less efficient than that of thefirst component, which is analyzed in transmission through thepolarizing beam splitter 4.

The bandpass filter 14 is used to spectrally purify the first componentof the beam and is provided in the first separate processing path F,while no such bandpass filter is provided to process the secondcomponent, because the polarization-selective effect of the polarizingbeam splitter 4, which is used to provide the spectral separation incombination with the spectrally selective retardation plate 12, is moreefficient in transmission than in reflection.

Since the second polarizing beam splitter 6 is located immediatelyadjacent the third LCoS panel 28 and its associated laminar element 26,the second polarizing beam splitter 6 performs first-stage analyzing ofthe reflected beam, to thereby provide a relatively high degree ofcontrast in the beam directed along the fourth processing path D.

Polarizer 24 is provided in the second processing path B to polarize thethird radiation component before entering the second polarizing beamsplitter 6. Again, the polarization state of the input beam I ispreferably linearly polarized in parallel with the polarization axis ofpolarizer 24. The second component is reflected in polarizing beamsplitter 6 by 90° to be directed along a third separate processing pathH to be modulated.

A skew angle compensator in the form of a quarterwave retarder 26processes the third component prior to reaching a third LCoS panel 28,at which the third component is modulated by selectively rotating thepolarization state of desired parts of the beam through 90°. Onreflection, desired parts of the third component are transmitted throughpolarizing beam splitter 6 and reflected from the reflective interfaceof dichroic beam splitter 8. Thus, desired parts of the first, secondand third component beams are combined and directed along the fifthprocessing path E. At this point, the first and second components areorthogonally polarized, whereas the first and third components exhibitparallel polarization states. However, unwanted parts of the first,second and third beam components remain at this stage, due to theimperfect analyzing performance of each of the two beam splitters 4, 6.In particular, the analyzing performance of the beam splitter 4 whenreflecting the desired components of the second component beam is moreimperfect than the performance of the two analyzers 4,6 in transmittingthe desired parts of the first beam component and the second beamcomponent, respectively.

A further analyzing stage is provided at the output part of the device,by means of three separate post analyzers 30, 32 and 34. Each analyzeris in the form of a spectrally-selective polarizer plate. The firstanalyzer 30 is selectively active in the part of the spectrumcorresponding to the wavelength range of the second spectral component,the second analyzer 32 is active in a spectral range corresponding tothe wavelength range of the first component, and the third analyzer 34is active in a spectral range corresponding to the wavelength range ofthe third component. Note that the analyzers 30, 32 and 34 may bearranged in any order.

As the desired parts of the first and second components are orthogonallypolarized when output along the fifth processing path E, the firstoutput analyzer 30 and the second output analyzer 32 have axes ofpolarization which are orthogonally arranged, such that the first outputanalyzer 30 has an axis of polarization which is parallel to that of thedesired part of the second component, and the second output analyzer 32has an axis of polarization which is parallel to the polarization stateof the desired part of the first component. Furthermore, the thirdoutput analyzer 34 has an axis of polarization which is arrangedparallel to that of the polarization state of the desired part of thethird component, and is also parallel to the axis of polarization of thesecond output analyzer 32.

Accordingly, the radiation emerging from the device at the output beam Ohas been subjected to second-stage analysis, provided by the threeseparate spectrally selective analyzers 30, 32 and 34.

Note that in the output beam O the polarization states of differentcomponents of the beam differ. In one embodiment, further elements inthe output part of the projection apparatus, for example the projectionlens, include one or more additional polarizing elements, which actsimilarly across the whole spectrum. In this embodiment, thepolarization direction of all of the three components are made parallelbefore reaching such additional polarizing elements by adding a furtherspectrally selective retardation plate to selectively rotate the secondlight beam component through 90° after passing through the threeanalyzers 30, 32 and 34.

FIG. 2 illustrates a further embodiment of the invention. In thisembodiment, a similar arrangement of the elements shown in FIG. 1, otherthan the output analyzers 30, 32 and 34, is also used. Similar referencenumerals are used in FIG. 2 for the common elements, and theirdescription will not be repeated for the sake of brevity. In place ofthe three output analyzers 30, 32 and 34, a furtherpolarization-sensitive stage of processing is provided by two processingelements 36 and 38. A spectrally selective retardation plate 36selectively rotates the polarization state of the second component, suchthat the desired parts of the second component have a polarization statewhich is parallel to the polarization state of the desired part of thefirst and third components. Subsequently, an analyzer plate 38, whichacts across the entire spectral range covered by the first, second andthird components, is provided to analyze the first, second and thirdcomponents together, to provide the second analyzing stage, therebyimproving contrast in the output beam O. An advantage of this embodimentis that the output beam has all of the three components with the samestate of polarization, so that if further polarizing components are usedin the remainder of the apparatus, all of the three components may besimilarly processed thereafter.

Each of the three cubic beam splitters 4, 6, 8 is preferably embodied inthe form of glass cubic components. Each of the remaining componentsillustrated in FIGS. 1 and 2 is a laminar component. All of thecomponents are bonded together, using an adhesive, in the arrangementshown in the Figures. In particular, the three cubic beam splitters 4, 6and 8 are all bonded together to form a unitary block, and each of theLCoS panels and its associated laminar elements is bonded in place onthe appropriate face of the cubic beam splitter. No air gap, nor anyadditional laminar films or other elements, are placed between the threecubic beam splitters 4, 6 and 8. The unitary block of the beam splittersprovides a rigid and stable body to be used for mounting LCoS panels,thereby providing improved convergence of the imaging provided by thepanels. By avoiding the positioning of the analyzers between the cubicbeam splitters, and instead locating the analyzer(s) on an external faceof the beam splitter arrangement, the device can be operated at highbrightness levels without degrading the analyzer(s), because theanalyzer(s) can be readily air-cooled. Furthermore, thermally inducedstress birefringence is reduced.

In each embodiment described, the second-stage analysis is provided by aplurality of laminar elements including a polarization-sensitiveelement. In one embodiment, the polarization-sensitive element is aspectrally selective retardation plate. In this respect, the term“polarization-sensitive” is intended to include all types of elementswhich have a desired effect, including both polarization-based filteringand polarization rotation, on the polarization state of the beam, whenarranged in the apparatus provided.

Herein the term “cubic beam splitter” is not intended to be limited tocubes with sides of equal length; the sides may have unequal lengths asdesired, in particular when the LCoS panels are themselves rectangular.

The above embodiments are to be understood as illustrative examples ofthe invention. Further embodiments of the invention are envisaged. It isto be understood that any feature described in relation to oneembodiment may also be used in other embodiments. Furthermore,equivalents and modifications not described above may also be employedwithout departing from the scope of the invention, which is defined inthe appended claims.

1. An optical device for processing radiation, said device comprising:a. radiation input means arranged to direct first and second spectralcomponents along a first processing path (A) and to direct a thirdspectral component along a second processing path (B); b. firstpolarization-selective reflective means (4) arranged to reflect thefirst spectral component and the second spectral component selectivelyin dependence on polarization states thereof, to direct said first andsecond components towards first and second radiation modulation means(18, 22), respectively, for modulation thereby and to direct the firstand second components, after modulation, along a third processing path(C), said first and second components having different polarizationstates when travelling along said third processing path; c. secondpolarization-selective reflective means (6) arranged to reflect thethird component selectively in dependence on a polarization statethereof, to direct said third component towards third radiationmodulation means (28), for modulation thereby and to direct the thirdcomponent, after modulation, along a fourth processing path (D); d.spectrally selective reflective means (8) arranged to process said firstand second spectral components similarly when in different polarizationstates, and arranged to direct said first and second components and saidthird component along a fifth processing path (E); and e. radiationoutput means (30, 32, 34; 36, 38) arranged to process radiation alongsaid fifth processing path, said output means including spectrallyselective polarization-sensitive means (30; 36) arranged to process saidfirst and second components differently when in different polarizationstates.
 2. An optical device as claimed in claim 1, wherein saidpolarization-sensitive means includes a first spectrally selectiveanalyzer (30) with an axis of polarization arranged parallel to apolarization state of the second component.
 3. An optical device asclaimed in claim 2, wherein said polarization-sensitive means includes asecond spectrally selective analyzer (32) with an axis of polarizationarranged parallel to a polarization state of the first component andperpendicular to said axis of polarization of the first analyzer.
 4. Anoptical device as claimed in claim 3, wherein saidpolarization-sensitive means includes a third spectrally selectiveanalyzer (34) with an axis of polarization arranged parallel to apolarization state of the third component and perpendicular to said axisof polarization of the first analyzer.
 5. An optical device as claimedin claim 1, wherein said polarization-sensitive means includes aspectrally selective polarization rotating element (36).
 6. An opticaldevice as claimed in claim 5, wherein said polarization-sensitive meansincludes an analyzer (38) arranged to process said first, second andthird components similarly when in a similar polarization state.
 7. Anoptical device as claimed in claim 1, wherein said input means comprisesselectively reflective means (2) arranged to direct the first and secondspectral components along the first processing path and to direct thethird spectral component along the second processing path by selectivereflection thereof.
 8. An optical device as claimed in claim 1, whereinsaid device further comprises spectrally selective polarization rotatingmeans (12) for rotating one of said first and second components inrelation to the other of the first and second components, arranged toprovide said first and second components with orthogonal polarizationstates when in said first processing path.
 9. An optical device asclaimed in claim 1, wherein said first and/or said secondpolarization-selective reflective means (4, 6) comprise a cubicpolarizing beam splitter.
 10. An optical device as claimed in claim 1,wherein said spectrally selective reflective means (8) comprise a cubicchromatic beam splitter.
 11. An optical device as claimed in claim 9,wherein said cubic beam splitters are bonded together to form a unitarycomponent.
 12. An optical device as claimed in claim 9 wherein saidcubic beam splitters are arranged without any analyzer elements arrangedbetween said cubic beam splitters.
 13. An optical device for processingradiation, said device comprising: a. radiation input apparatus arrangedto direct first and second spectral components along a first processingpath and to direct a third spectral component along a second processingpath; b. a first polarization-selective reflective element arranged toreflect the first spectral component and the second spectral componentselectively in dependence on polarization states thereof, to direct saidfirst and second components towards first and second radiationmodulation devices, respectively, for modulation thereby and to directthe first and second components, after modulation, along a thirdprocessing path, said first and second components having differentpolarization states when travelling along said third processing path; c.a second polarization-selective reflective element arranged to reflectthe third component selectively in dependence on a polarization statethereof, to direct said third component towards a third radiationmodulation device, for modulation thereby and to direct the thirdcomponent, after modulation, along a fourth processing path; d. aspectrally selective reflective element arranged to process said firstand second spectral components similarly when in different polarizationstates, and arranged to direct said first and second components and saidthird component along a fifth processing path; and e. radiation outputapparatus arranged to process radiation along said fifth processingpath, said output apparatus including a spectrally selectivepolarization-active element arranged to process said first and secondcomponents differently when in different polarization states.